1. Field of the Invention
The invention relates to a method and apparatus for controlling the diameter of crystals grown by the Czochralski process.
2. Description of the Prior Art
The Czochralski process has, for many years, been used to grow single crystals from a melt of the crystal material. The melt is contained in a crucible into which an appropriately oriented seed is dipped. After the seed has reached thermal equilibrium, it is slowly withdrawn causing the formation of new crystalline structure at the interface of the crystal and the melt. The rate of crystal growth depends on the balance between the heat lost from the crystal by radiation and convection and the heat input from the melt to the crystal. Under steady state conditions, the rate of crystal growth equals the rate of withdrawal from the melt. An increase in heat input or withdrawal rate of the crystal diminishes the diameter, while a decrease in heat input or the withdrawal rate of the crystal increases the diameter.
Prior art methods having attempted to control the diameter of crystals grown by the Czochralski method by controlling either the temperature of the melt or the rate at which the crystal is withdrawn from the melt. Temperature of the melt and the withdrawal rate of the crystal have been controlled in relation to a human operator's observation of deviations in the diameter of the crystal and in relation to thermostatic measuring devices.
Although the control of the crystal diameter by human operators is adequate for some types of crystals, the physical properties of other types of crystals demand an automated diameter control method. Where the method of controlling the crystal diameter had been to regulate the withdrawal rate of the crystal, the thermostatic measuring devices have included infrared sensors. However, since the prior art methods of controlling the withdrawal rate of the crystal require precise mechanical control, these methods have been expensive and have not provided the degree of accuracy required for the growth of certain types of crystals for example, oxide-type optical materials. Methods which regulate the diameter of the crystal by controlling the temperature of the melt have not used infrared sensors and have not been sufficiently responsive for accurate control of the crystal diameter. Therefore, there was a need for a more accurate method of controlling the diameter of crystals grown by the Czochralski process.